Microcellular polyurethane elastomers are well known. They have fine, evenly distributed cells, and densities that are low compared with solid urethane elastomers yet high compared with flexible polyurethane foam. Microcellular polyurethane elastomers are used in automotive parts (e.g., bumpers and armrests), gaskets, vibration damping applications, and footwear.
While many ways to make microcellular polyurethane elastomers have been revealed, most approaches fit into two categories: the "one-shot" method and the "prepolymer" method. In the one-shot method, all of the components (polyols, polyisocyanate, blowing agents, surfactant, catalyst, chain extenders) are combined and reacted in a single step. In contrast, the prepolymer approach pre-reacts the polyisocyanate with a polyol to make a "prepolymer" (the "A" side) that is subsequently combined with the remaining reactants including any chain extenders (the "B" side), in a second step to make the elastomer. As U.S. Pat. No. 4,559,366 illustrates, it can be beneficial to make a "quasiprepolymer" by using an by reacting the polyol with enough polyisocyanate to produce a mixture of isocyanate-terminated prepolymer and free polyisocyanate. Such quasiprepolymers are commonly used to boost the available NCO content of the "A" side.
It is also known to prepare prepolymers ("A" sides) from isocyanates and polyol-chain extender mixtures. For example, U.S. Pat. No. 5,658,959 teaches to make an isocyanate-terminated prepolymer from MDI, dipropylene glycol, a polyoxypropylated/ethoxylated glycerine, and a polyoxypropylated/ethoxylated glycol (see Example 1). The reference polyols have up to 35 wt. % of ethylene oxide content, but an undisclosed degree of "endcapping" or primary hydroxyl group content (see column 5, lines 17-38). The reference is also silent regarding the unsaturation level of the polyols. U.S. Pat. No. 5,618,967 contains a similar disclosure. In sum, these references suggest that neither the unsaturation level nor the primary hydroxyl content of the polyols is important.
U.S. Pat. No. 5,284,880 also shows (see, e.g., column 13, lines 30-45) a prepolymer made from an isocyanate, a polyol, and a chain extender (dipropylene glycol). This reference teaches, however, that the "A" side polyol must be a "polyether containing predominately secondary hydroxyl groups" (see Abstract; col. 2, lines 4-5; and col. 4, lines 28-54). This reference is also silent regarding any need for a low-unsaturation polyol.
The benefits of polyols with low levels of unsaturation (&lt;0.020 meq/g) for polyurethanes generally and for microcellular polyurethane elastomers in particular are known. U.S. Pat. Nos. 5,677,413 and 5,728,745, for example, describe microcellular polyurethanes made from polyols having unsaturations less than about 0.010 meq/g. The '745 patent makes the elastomers by either the prepolymer method (see Example 8 and Table 6 of the reference) or by the one-shot approach (see Examples 9-11 and Table 8 of the reference). The prepolymers of Example 8 are reaction products of polyoxypropylene diols or triols with 4,4'-MDI. No chain extender is used to make the prepolymer. In Examples 9-11, high-primary, low-unsaturation polyols are used. The references teach several advantages of using low-unsaturation polyols, including good resilience, low compression set, and reduced shrinkage; these advantages are particularly important for shoe soles.
U.S. Pat. No. 5,106,874 teaches prepolymer and one-shot approaches to making noncellular elastomers from low-unsaturation polyols. The prepolymers are generally made by reacting polyoxyalkylene polyols with an excess of polyisocyanate. The reference teaches that chain extenders can be included in the prepolymer (column 7, lines 49-52). However, none of the actual examples includes a chain extender reacted into the "A" side, and no microcellular elastomers are made.
U.S. Pat. No. 5,696,221 teaches to make polyurethane/urea elastomers by reacting prepolymers with a chain extender. The prepolymers include a diol having a molecular weight less than 400 in addition to a low-unsaturation, polyoxypropylene diol. The reference does not disclose microcellular elastomers.
Despite the well-recognized benefits of using low-unsaturation polyols in formulating microcellular polyurethane elastomers, some problems remain with the conventional one-shot and prepolymer approaches. As noted in U.S. Pat. No. 4,559,366, the one-shot approach cannot easily be used with 4,4'-diphenylmethane diisocyanate (4,4'-MDI), a ubiquitous raw material for shoe sole elastomers, because it is not readily miscible with other reactants, and it solidifies at room temperature (see col. 1 of the reference).
The prepolymer approach, however, also has drawbacks. Formulating high-quality, low-density elastomers, especially ones that have densities less than 0.5 g/cm.sup.3, is difficult. An obvious way to reduce density is to increase the amount of blowing agent (usually water). However, this increases the urea content of the elastomer, reduces elongation, and reduces flexibility. Adding more chain extender into the "B" side helps to maintain good hardness at lower densities, but this can cause poor processability and premature phase separation. As Comparative Example 8 (below) shows, such products often have an undesirable incidence of surface defects and internal splitting.
While it is known to include some chain extender in the "A" side, little or nothing is known about the benefits of doing so in the context of making microcellular elastomers based on low-unsaturation polyols, particularly those having a high content of primary hydroxyl groups.
In sum, the industry would benefit from better ways to make microcellular polyurethane elastomers, especially low-density elastomers. A preferred approach would use the low-unsaturation polyols now known to confer significant physical property advantages to urethanes. A valuable process would be easy to practice, yet would overcome the drawbacks of the conventional one-shot and prepolymer methods, particularly in formulating low-density elastomers.